D. Bosnar

Search at the Mainz Microtron for light massive gauge bosons relevant for the muon g-2 anomaly.

H. Merkel,1, ∗ P. Achenbach,1 C. Ayerbe Gayoso,1 T. Beranek,1 J. Beričič,2 J. C. Bernauer,1, † R. Böhm,1 D. Bosnar,3 L. Correa,1 L. Debenjak,2 A. Denig,1 M. O. Distler,1 A. Esser,1 H. Fonvieille,4 I. Friščić,3 M. Gómez Rodrı́guez de la Paz,1 M. Hoek,1 S. Kegel,1 Y. Kohl,1 D. G. Middleton,1 M. Mihovilovič,1 U. Müller,1 L. Nungesser,1 J. Pochodzalla,1 M. Rohrbeck,1 G. Ron,5 S. Sánchez Majos,1 B. S. Schlimme,1 M. Schoth,1 F. Schulz,1 C. Sfienti,1 S. Širca,2, 6 M. Thiel,1 A. Tyukin,1 A. Weber,1 and M. Weinriefer1 (A1 Collaboration) Institut für Kernphysik, Johannes Gutenberg-Universität Mainz, D-55099 Mainz, Germany‡ Jožef Stefan Institute, SI-1000 Ljubljana, Slovenia Department of Physics, University of Zagreb, HR-10002 Zagreb, Croatia Clermont Université, Université Blaise Pascal, CNRS/IN2P3, LPC, BP 10448, F-63000 Clermont-Ferrand, France Racah Institute of Physics, Hebrew University of Jerusalem, Jerusalem 91904, Israel Department of Physics, University of Ljubljana, SI-1000 Ljubljana, Slovenia (Dated: April 22, 2014)

Observation of H Λ 4 Hyperhydrogen by Decay-Pion Spectroscopy in Electron Scattering

At the Mainz Microtron MAMI, the first high-resolution pion spectroscopy from decays of strange systems was performed by electron scattering off a (9)Be target in order to study the Λ binding energy of light hypernuclei. Positively charged kaons were detected by a short-orbit spectrometer with a broad momentum acceptance at 0° forward angles with respect to the beam, efficiently tagging the production of strangeness in the target nucleus. Coincidentally, negatively charged decay pions were detected by two independent high-resolution spectrometers. About 10(3) pionic weak decays of hyperfragments and hyperons were observed. The pion momentum distribution shows a monochromatic peak at pπ≈133u2009u2009MeV/c, corresponding to the unique signature for the two-body decay of hyperhydrogen Λ(4)H→(4)He+π(-), stopped inside the target. Its Λ binding energy was determined to be BΛ=2.12±0.01u2009u2009(stat)±0.09u2009u2009(syst)MeV with respect to the (3)H+Λ mass.

Exclusive electroproduction of K+Λ and K+Σ0 final states at Q2 = 0.030–0.055 (GeV/c)2

Cross-section measurements of the exclusive p(e, e′K+)Λ, Σ0electroproduction reactions have been performed at the Mainz Microtron MAMI in the A1 spectrometer facility using for the first time the KAOS spectrometer for kaon detection. These processes were studied in a kinematical region not covered by any previous experiment. The nucleon was probed in its third-resonance region with virtual photons of low four-momenta, Q2 = 0.030–0.055 (GeV/c)2. The MAMI data indicate a smooth transition in Q2 from photoproduction to electroproduction cross-sections. Comparison with predictions of effective Lagrangian models based on the isobar approach reveal that strong longitudinal couplings of the virtual photon to the N* resonances can be excluded from these models. Modern isobar and Regge-plus-resonance models are in agreement with the data.

Final State Interaction Effects in pol 3He(pol e,e'p)

Asymmetries in quasi-elastic pol 3He(pol e,ep) have been measured at a momentum transfer of 0.67 (GeV/c)^2 and are compared to a calculation which takes into account relativistic kinematics in the final state and a relativistic one-body current operator. With an exact solution of the Faddeev equation for the 3He-ground state and an approximate treatment of final state interactions in the continuum good agreement is found with the experimental data.

First measurement of proton's charge form factor at very low Q2 with initial state radiation

Abstract We report on a new experimental method based on initial-state radiation (ISR) in e–p scattering, which exploits the radiative tail of the elastic peak to study the properties of electromagnetic processes and to extract the proton charge form factor ( G E p ) at extremely small Q 2 . The ISR technique was implemented in an experiment at the three-spectrometer facility of the Mainz Microtron (MAMI). This led to a precise validation of radiative corrections far away from elastic line and provided first measurements of G E p for 0.001 ≤ Q 2 ≤ 0.004 ( GeV / c ) 2 .

Measurement of the Strong Interaction Induced Shift and Width of the 1

The antikaon-nucleon interaction close to threshold provides crucial information on the interplay between spontaneous and explicit chiral symmetry breaking in low-energy QCD. In this context the importance of kaonic deuterium X-ray spectroscopy has been well recognized, but no experimental results have yet been obtained due to the difficulty of the measurement. We propose to measure the shift and width of the kaonic deuterium 1s state with an accuracy of 60 eV and 140 eV respectively at J-PARC. These results together with the kaonic hydrogen data (KpX at KEK, DEAR and SIDDHARTA at DAFNE) will then permit the determination of values of both the isospin I=0 and I=1 antikaon-nucleon scattering lengths and will provide the most stringent constraints on the antikaon-nucleon interaction, promising a breakthrough. Refined Monte Carlo studies were performed, including the investigation of background suppression factors for the described setup. These studies have demonstrated the feasibility of determining the shift and width of the kaonic deuterium atom 1s state with the desired accuracy of 60 eV and 140 eV.

s

In the exotic atoms where one atomic 1s electron is replaced by a K − , the strong interaction between the K − and the nucleus introduces an energy shift and broadening of the low-lying kaonic atomic levels which are determined by only the electromagnetic interaction. By performing X-ray spectroscopy for Z = 1,2 kaonic atoms, the SIDDHARTA experiment determined with high precision the shift and width for the 1s state of K − p and the 2p state of kaonic helium-3 and kaonic helium-4. These results provided unique information of the kaon-nucleus interaction in the low energy limit.

State of Kaonic Deuterium at J-PARC

We report new p(e, e′p)π° measurements in the Δ+(1232) resonance at the low momentum transfer region utilizing the magnetic spectrometers of the A1 Collaboration at MAMI. The mesonic cloud dynamics are predicted to be dominant and appreciably changing in this region while the momentum transfer is sufficiently low to be able to test chiral effective field theory calculations. The results disagree with predictions of constituent quark models and are in reasonable agreement with dynamical calculations with pion cloud effects, chiral effective field theory and lattice calculations. The reported measurements suggest that improvement is required to the theoretical calculations and provide valuable input that will allow their refinements.

Precision X-ray spectroscopy of kaonic atoms as a probe of low-energy kaon-nucleus interaction

We have measured the proton recoil polarization in the 4He(e-->,e()p-->)4H reaction at Q(2)=0.5, 1.0, 1.6, and 2.6 (GeV/c)(2). The measured ratio of polarization transfer coefficients differs from a fully relativistic calculation, favoring the inclusion of a medium modification of the proton form factors predicted by a quark-meson coupling model. In addition, the measured induced polarizations agree reasonably well with the fully relativistic calculation indicating that the treatment of final-state interactions is under control.Polarization transfer in the 4HeH reaction at a Q2 of 0.4xa0(GeV/c)2 was measured at the Mainz Microtron MAMI. The ratio of the transverse to the longitudinal polarization components of the ejected protons was compared with the same ratio for elastic ep scattering. The results are consistent with a recent fully relativistic calculation which includes a predicted medium modification of the proton form factor based on a quark–meson coupling model.We have measured the proton recoil polarization in the {sup 4}He(polarized-e, e-prime, p){sup 3}H reaction at Q{sup 2} = 0.5, 1.0, 1.6, and 2.6 (GeV/c){sup 2}. The measured ratio of polarization transfer coefficients differs from a fully relativistic calculation, favoring the inclusion of a predicted medium modification of the proton form factors based on a quark-meson coupling model. In contrast, the measured induced polarizations agree reasonably well with the fully relativistic calculation indicating that the treatment of final-state interactions is under control.Polarization transfer in the 4He(e,ep)3H reaction at a Q^2 of 0.4 (GeV/c)^2 was measured at the Mainz Microtron MAMI. The ratio of the transverse to the longitudinal polarization components of the ejected protons was compared with the same ratio for elastic ep scattering. The results are consistent with a recent fully relativistic calculation which includes a predicted medium modification of the proton form factor based on a quark-meson coupling model.

Measurements of the γ*p → Δ reaction at low Q2

Abstract We report the first measurements of the transverse ( P x and P y ) and longitudinal ( P z ) components of the polarization transfer to a bound proton in the deuteron via the H 2 ( e → , e ′ p → ) reaction, over a wide range of missing momentum. A precise determination of the electron beam polarization reduces the systematic uncertainties on the individual components to a level that enables a detailed comparison to a state-of-the-art calculation of the deuteron using free-proton electromagnetic form factors. We observe very good agreement between the measured and the calculated P x / P z ratios, but deviations of the individual components. Our results cannot be explained by medium modified electromagnetic form factors. They point to an incomplete description of the nuclear reaction mechanism in the calculation.